Unlimited Metrocard fares go up near the end of the month (Sunday, February 27), and the MTA is giving riders a grace period with cards bought at pre-2005 fare hike prices. But not too much of a grace period, so people won't really be able to hoard cards:
- Weekly Metrocards, purchased at $21, are good until March 7
- Monthly Metrocards, purchased at $70, can be used until April 3 only

After those dates, that little black strip on the Metrocard will say your card is not usable. And to be customer-friendly, the MTA is offering refunds on a "pro-rated" basis; "Ask for a postage-paid envelope at your station booth to return your card." So, if you haven't purchased your monthly unlimited card yet, you might want to wait until February 26. Unless you're a really heavy subway user - then needing to buy unlimited weekly cards in the interim might not be worth it.

MTA on the new fares for Metrocards. Oh, and expect more fare increases on the horizon.